Centipede slow wave circuit and microwave tubes using same



Oct. 6, 1970 T.'-RoUMBANls 3,532,924

CENTIPEDE SLOW WAVE CIRC- IT vAND MICROWAVE TUBES USING-SAME Filed May 2o, 196e 'i vINVENTOR T HEonoREy lfzoumnms'V ATTORNEY United States Patent O U.s. el. S-3.5 3 claims ABSTRACT OF THE DISCLOSURE A microwave amplifier tube is disclosed. The amplifier tube includes-an electron gun for forming and projecting a stream of electrons over an elongated beam path to a collector electrode disposed at the terminal end of the tube. A centipede slow wave circuit is disposed along the the beam path for cumulative electromagnetic interaction with the electron beam to produce an amplified output microwave signal which is extracted from the circuit and fed to a suitable utilization device. The centipede slow wave circuit is formed by a succession of discs transversely mounted in axially spaced relation within a conductive tube. Each of the discs includes an array of radially directed leg portions with adjacent leg portions formed into an S and a reverse S-shape, respectively, to provide negative mutual inductive coupling between adjacent coupled cavity sections defined by the spaces between adjacent discs. The leg portions are made of a constant width over their radial length to facilitate fabrication of the centipede slow .wave circuit.

DESCRIPTION OF THE PRIOR ART The centipede slow wave circuit was disclosed and claimed in U.S. Pat. 3,233,139 issued Feb. 1, 1966 and assigned to the same assignee as the present invention. The centipede slow wave circuit is characterized by negative mutual inductive coupling between adjacent coupled cavities. Such a slow wave circuit has a fundamental forward wave dispersion characteristic and provides increased efficiency and bandwidth as compared with coupled cavity slow wave circuits having positive mutual inductive coupling between adjacent cavities. This results because such positive inductively coupled circuits are a fundamentally backward wave circuit and must be operated on the first space harmonic to obtain forward wave interaction. First space harmonic interaction results in reduced interaction impedance as compared to a fundamental forward wave circuit.

In the first centipede slow wave circuits, the coupled cavities were defined by the spaces between successive transverse discs carried within a tubular conductor. The discs were centrally apertured to accommodate the beam passable therethrough. Negative mutual inductive coupling was provided by alternate S and reverse S-shaped legs provided around the periphery of each of the discs. The legs were formed by pie-shaped sectors of the disc.

It was subsequently discovered that the bandwidth of the centipede slow wave circuit could be increased by drilling an array of holes through each of the discs to open up the spaces between adjacent legs intermediate their length. Such a structure is described in an article titled A Structure Using Resonant Coupling Elements, Suitable For A High-Power Travelling-Wave Tube by A. F. Pearce, appearing in the Proceedings of the Institution of Electrical Engineers, vol. 105, Part B, Supplement No. l1, London, May 1958, pp. 719-726. The individual legs, formed into the S-shape, provided the inductive coupling loops. The resonant frequency of the in- 3,532,924 Patented Oct. 6, 1970 ductive coupling loops was determined by the inductance of each loop and by the capacitance between adjacent loops or legs. Drilling the axially directed holes through the spaces between adjacent legs opened up the space between adjacent legs and increased the resonance frequency of the loops, thereby increasing the resonant 1rmode frequency of the slow wave circuit. This thereby raised the high frequency cutoff of the -fr-mode and increased the bandwidth of the circuit.

It was found still later that the centipede slow wave circuit could be more easily fabricated by causing the disc-shaped members to be formed in two parts. More specifically, each disc comprised a sandwich of two similar disc-shaped members with each disc having an array of similarly shaped leg portions. The two discs were then placed together such that the leg portions were interdigitated to provide the alternate S and reverse S-shaped coupling loops around the periphery of the sandwiched disc structures. However, when the discs were constructed in this manner, it was found that it was extremely difficult to hold the proper spacing between adjacent interdigitated legs, especially at the point where they were inwardly notched for decreasing the capacitance between adjacent legs.

Therefore, a need exists for providing an improved centipede disc structure which will be relatively easy to fabricate and which will provide constant capacitance between adjacent inductive coupling legs or loops.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of an improved centipede slow wave circuit and microwave tubes using same.

One feature of the' present invention is the provision, in a centipede slow wave circuit, of negative mutual inductive coupling leg portions of constant width forming coupling loops, whereby the centipede slow wave circuit is more easily fabricated and whereby the capacitance between adjacent inductive coupling legs is more easily held at a constant value.

Another feature of the present invention is the same as the preceding feature wherein the centipede disc structures are formed by a sandwiched construction of a pair of similarly shaped discs disposed in mutually opposed relation with the leg portions of one disc interdigitated with the leg portions of the adjacent discs.

Another feature of the present invention is the same as any one or more of the preceding features wherein the coupling loop dimensions of successive discs of the centipede slow wave circuit are progressively changed taken in a direction toward the end of the circuit to progressively increase the resonant frequency of the coupling loops in the few terminal coupled cavities to provide a wideband match to the load to prevent bandedge oscillations.

Another feature of the present invention is the same as the preceding feature wherein the axial spacing between successive disc structures decreases taken in the direction along the beam path toward the terminal end of the slow wave circuit for decreasing the phase velocity of the circuit to match the decreased velocity of the electron bunches to provide increased electronic interaction with the beam.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. l is a longitudinal View, partly in section and partly schematic, of a microwave tube employing features of the present invention,

FIG. 2 is an enlarged longitudinal sectional View of a portion of the structure of FIG. l delineated by line 2-2,

FIG. 3 is a sectional view of the structure of FIG. '2 taken along line 3 3 in the direction of the arrows,

FIG. 4 is a plan view of one of the disc members employed to form the centipede disc structure and depicting the prior art leg portions in phantom, and

FIG. 5 is an w-,B diagram for the circuit of FIGS. 2 and 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. `1, there is shown a microwave tube 1 incorporating features of the present invention. The tube 1 includes an electron gun assembly 2 for forming and projecting a beam of electrons over an elongated beam path 4 to a collector structure 3 disposed at the terminal end of the beam path 4. An electromagnetic interaction circuit 5 is disposed along the beam path for electromagnetic interaction with the beam to produce an amplified output microwave signal which is extracted from the downstream end of the interaction circuit S via an output waveguide 6 and fed to a suitable load or utilization device, not shown. Signals to be amplified are fed into the upstream end of the interaction circuit 5 via an input coaxial line 7.

The interaction circuit S can take several forms. However, it generally includes a buncher circuit portion 8 followed by an output circuit portion 9. In the present invention at least one of the circuit portions 8 or 9 comprises a centipede slow wave circuit. If the microwave tube 1 is a conventional traveling wave tube, both the buncher and output circuits 8 and 9 preferably comprise the centipede type slow wave circuit. In such a case, the buncher circuit 8 is terminated, at its downstream end, in a resistive termination and the upstream end of the output circuit 9 is terminated in a resistive termination, both terminations being schematically indicated at 1,1. These resistive terminations 11 may be located internal of the A vacuum envelope of the tube or may be external to the vacuum envelope of the tube. A beam focus solenoid 12 surrounds the interaction circuit 5 for producing an axially directed beam focus magnetic iield throughout the interaction circuit 5 for coniining the electron beam to a desired diameter throughout the length of the circuit 5.

In the case of a hybrid type tube, the buncher circuit 8 may comprise, for example, a succession of klystron type cavity resonators with the resonators tuned to the band edges of the operating band of the tube, whereas the output circuit 9 would comprise the centipede slow wave circuit providing a peaked response within the center band of the tube, such that the composite gain of the entire tube is flat over an extremely wide band of frequencies as of Both of the aforementioned types of microwave tubes are disclosed and claimed in U.S. Pat. 3,374,390 issued Mar. 19, 1968 and assigned to the same assignee as the present invention.

Referring now to FIGS. 2-4, the improved centipede slow wave circuit 9 will be described in greater detail. The centipede slow wave circuit 9 includes a hollow cylindrical conductor 13 as of copper having a succession of conductive disc structures 14, as of copper, axially spaced apart and transversely mounted within the conductive tube 13 along the length thereof. The spaces between the conductive discs `,14 which are bounded, on the sides, by the inside wall of the tubular conductor 13 and bounded, on the ends, by adjacent discs 14 deine a succession of coupled cavity structures 15. The disc structures 14 each include a central aperture 16 which is axially aligned with the beam path 4 for passage of the beam therethrough.

Each of the disc structures 14 includes an outer peripheral portion 17 which is radially segmented to form an array of radially directed leg portions 18. Adjacent leg portions 18 are formed into an S and a reverse S coniiguration, respectively, to provide negative mutual inductive coupling loops coupling wave energy between adjacent coupled cavity sections 15, in the manner as previously disclosed in the aforecited U.S. Pat. 3,233,139 and in the aforecited Proceedings of the Institution of Electrical Engineers article. A tubular coolant jacket 19, as of copper, is disposed surrounding the tubular conductor 13 in heat exchanging relation therewith for Icollecting heat from the disc structures 14 and wall structure 14 into the coolant jacket 19 wherein the thermal energy passes into a fluid coolant circulating through a helical coolant channel 21. In this manner, thermal energy is removed from the circuit 9 to prevent overheating thereof in use.

The centipede disc structures 14 are preferably formed by a pair of similarly shaped disc members of the type generally indicated by the solid lines in FIG. 4. Each of the disc members comprises one-half of the total number of inductive coupling legs 18. Each similarly shaped disc member is formed with the same shape for all of its legs and two such disc members are then placed together in mutually opposed relation with their leg portions 18 interdigitated to provide the adjacent S and reverse S- shaped coupling loop portions 18 near the periphery 17 of the disc structures 14.

The conductive tube 13 is formed by a succession of axially aligned ring segments 24 which serve to capture the free ends of the legs 18 between adjacent ends of the rings 24. In addition, a retaining ring 22, as of copper, and having a thickness equal to the thickness of the legs 18 is positioned between adjacent ring segments 24. The retaining ring 22 is provided with an array of rectangular notches 23, there being one notch 23 for each of the total number of leg portions 18 radially projecting from a single disc structure 14. However, since only one-half of the total radial leg portions 18 on one disc extend on each side of each disc structure 14, every other registering notch 23, in any given retaining ring 22, receives and indexes one-half of the legs 118 or one disc structure 14, whereas the intervening notches serve to register the legs from the next adjoining disc structure 14. In this manner, total registry of all of the legs 18 is achieved throughout the length of the slow wave circuit 9.

Referring now to FIGS. 3 and 4, it is seen that the leg portions have a uniform constant width throughout their length as contrasted with the prior art pie-shaped legs, indicated in dotted lines in FIG. 4. The leg portions 18, having uniform thickness, are more easily fabricated since the straight-sided V-shaped notices between adjacent legs 18 are more easily cut, as by milling, than the notches having outwardly directed portions as indicated by the space between adjacent pie-shaped legs 1,8 shown in dotted lines. Aside from being easier to manufacture, the uniform width legs 1-8 facilitate maintaining constant spacing between adjacent legs 18 when they are interdigitated in the manner indicated in FIGS. 2 and 3. The peripheral spacing between the crossover points of the interdigitated legs determines the capacitance of the inductive coupling loop portions formed by the S and reverse S-shaped legs 18. In the prior art, the notches in the pie-shaped legs, intermediate their length, were provided to reduce the capacity between adjacent legs at their crossover points. However, due to the pie-shaped nature of the prior art legs 18', when they were bent into the S shape, a slight deformation of the S-shaped loop produced a variation in the capacitance between adjacent legs. On the other hand, the uniform width leg 18 makes the spacing between adjacent legs at their crossover points much less critical with regard to deformation of the S-shaped loops.

The leg portions 18, of uniform width, also facilitate forming the S-shaped and reverse S-shaped coupling loops with atter configurations to obtain a phase velocity taper and a taper of the upper bandedge frequencies for the slow wave circuit in order to improve electronic interaction and to prevent bandedge oscillations. More specifically, as microwave energy is extracted from the modulated beam, the beam velocity decreases. In order to maintain a near constant interaction impedance with the beam, as it slows in velocity, the phase velocity of the circuit 9 is decreased. This is accomplished by decreasing the period L of the cavity sections over the last few cavities at the downstream end of the circuit 9. As seen in FIG. 2, the axial length per period, L1, I4 L5, successively decreases toward the downstream end of the circuit 9. This permits the circuit wave to have a phase velocity which remains in sychronism with the reduced velocity of the beam as indicated in the w-,B diagram of FIG. 5. As the period L is reduced in length near the downstream end of the circuit 9, this requires that the shape of the S and reverse S-shaped loops be flattened to accommodate the reduced period L.

In addition, it is desirable to increase the upper cutoft` bandedge frequency for the cavities 15 in accordance with the decrease in the period L, as shown in FIG. 5, to produce a frequency taper of the upper cutoi frequency to obtain an improved impedance match between the circuit 9 and the load for preventing bandedge oscillations. This method for preventing bandedge oscillations forms the subject matter of and is claimed in copending U.S. application 516,939, filed Dec. 28, 1965, now U.S. Pat. 3,414,756, issued Dec. 3, 1968 and assigned to the same assignee as the present invention. Briefly, the upper cutoff frequency for the successive resonator sections 15 is increased by flattening the area of the S and reverse S-shaped coupling loops in toward the center plane of the disc structures 14. The uniform width of the fingers 18 facilitates shaping of the coupling loops since it changes the crossover point without substantially changing the capacity between adjacent legs 18. The inside diameter of the few terminal cavity sections is progressively decreased to offset low bandedge frequency changes produced by the flattened loops.

In a typical example of the present invention, an S- band centipede slow wave circuit 9, incorporating features of the present invention, provided a cold bandwidth from 2.9 ggahertz to 3.9 ggahertz and when incorporated into a tube 1 it provided an instantaneous bandwidth of 20%.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. vIn a microwave tube apparatus, means for forming and projecting a beam of electrons over an elongated beam path, collector means at the terminal end of the beam path for collecting and dissipating energy of the beam, a centipede slow vwave circuit disposed along the beam path for cumulative electromagnetic, interaction between microwave energy on said circuit and the electrons of the beam, said centipede slow wave circuit comprising a hollow conductive tube structure having an array of axially spaced centrally apertured conductive disc structures transversely mounted therein to define a coupled cavity slow wave structure, said conductive tube and said disc being coaxially aligned with the beam path and the beam path passing successively through the aligned central apertures in said discs of said array, each of said discs having an array of peripherally spaced radially directed leg portions with adjacent ones of said leg portions being of S-shape and a reverse S-shape, respectively, to provide negative mutual inductive coupling loops between adjacent coupled cavities, said centipede slow wave circuit having a forward wave fundamental mode space harmonic dispersion characteristic, the improvement wherein, said S and reverse S-shaped loop portions are attened more into the plane of the discs within a few of said coupled cavities of said slow wave circuit which are disposed near at least one end of said slow wave circuit, to increase the high frequency cutoff for these few terminal coupled cavities and to provide a wideband match to a load for preventing bandedge oscillations.

2. The apparatus of claim 1 wherein each of said disc structures is formed by a pair of similar disc structures positioned in mutually opposed abutting relation at their central regions with the leg portions of one of said discs interdigitated with the leg portions of the other one of said discs, the leg portions of one of said disc structures providing the S-shaped coupling loops and the leg portions of the other one of said discs providing the reverse S-shaped coupling loop portions.

3. The apparatus of claim 1 wherein the axial spacing between adjacent discs decreases taken in the direction along said circuit toward the downstream end of said slow wave circuit and within a few coupled cavity sections near the downstream end of said circuit, for decreasing the phase velocity of wave energy on the circuit to provide increased electronic interaction with the beam.

References Cited UNITED STATES PATENTS Re. 25,329 2/1963 Bates S15-39.3 X 3,233,139 l/l-966 Chodorow 3l5--3.5 3,374,390 3/1968 Ruetz et al. S15-39.3 X

OTHER REFERENCES Power Travelling-Wave Tubes, by I. F. Gittins, TK 7872 TG5, C. 2, 1965, pages 80 and 81 relied upon.

ELI LIEBERMAN, Primary Examiner S. CHATMON, JR., Assistant Examiner U.S. Cl. X.R. 

